Studies are being conducted of the transmission of recessive lethal mutations which were induced in mice by chemical mutagens. Backcross progeny between offspring of lethal bearing animals and their daughters are used to detect the presence of recessive lethals. An increase in fetal mortality is used as indicator of the lethal. The induction of recessive lethal mutations by a chemical has previously been demonstrated in this project. No adequate study of this class of mutation has been conducted with regard to heterozygous effects. It is our purpose to investigate what might be the mode of action and long-term effects of such mutations on mammals.